Molecule: a molecule is the smallest particle in a chemical element or compound that has the chemical properties of that element or compound. Molecules are made up of two or more atoms that are held together by chemical bonds. These bonds form as a result of the sharing or exchange of electrons among said atoms. Although oxygen is an element, it naturally has 2 atoms (O2), because it is a halogen and in order to reach 8 valence electrons and become stable and unreactive, it bonds with another oxygen atom. This means that oxygen atoms and oxygen molecules are different; one is an element, the other is a molecule.
Pure Substance: can be divided into two sub-categories: compounds and elements.
Compound: A chemical compound is a chemical substance composed of many identical molecules composed of atoms from more than one element held together by chemical bonds. A molecule consisting of atoms of only one element is therefore not a compound, meaning that oxygen molecules (O2) are NOT compounds. NaCl, or table salt, is.
Atom: An atom is the smallest unit of ordinary matter that forms a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms.
Element: An element is a pure substance consisting only of atoms that all have the same numbers of protons in their atomic nuclei. Elements are the base, more "pure" of the pure substances. Each element is assigned a symbol. Perhaps the most recognizable of them is H, for hydrogen.
Organic Compounds: organic compounds are generally any chemical compounds that contain carbon-hydrogen bonds. The study of these is called organic chemistry. I mean, carbon is the basis of ALL known life.
Inorganic Compounds: They are any substance in which two or more chemical elements (usually other than carbon) are combined, nearly always in definite proportions. Compounds of carbon are classified as organic when carbon is bound to hydrogen. THE OPPOSITE OF ORGANIC COMPOUNDS.
Chemistry is fun!! Hope this helps!! Have a wonderful day!!